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Topic: mosfet failure reasons? (Read 3903 times) previous topic - next topic

winner10920

What can cause a nchannel mosfet(irf510) to fail?
I've gone thru 2, the first one acted as a short but always did from the start
the second one worked for a little while then went dead(not nearly as loud and strange waveform even driven direct dc) sometime while I was moving the speaker it was driving
I have a 555 timer driving it directly at 5khz
I have it in a push pull configuration with a pchannel mosfet, tho that's not hooked up to v - yet

DVDdoug

Usually, too much current or too-much power-dissipation.   

Quote
I have it in a push pull configuration with a pchannel mosfet, tho that's not hooked up to v - yet
What is connected?     Do you have a schematic, or a link to your schamatic?  Something might be wired wrong...

You can more power dissipation if you use it in an "analog" mode (or accicently) to turn it partially-on so that it has current through-it, and voltage across it, at the same time.   When used as a switch, which you should be doing with a (digital) 555, it's either off (with voltage across it but almost no current) or on (with current through-it, but with most of the voltage across the load),  it shouldn't get hot (or dissipate power)

be80be

Sounds like you have shoot-through show your circuit. But its hard to say not seeing how you have the speaker hooked up. But you should be using a cap to feed the speaker.

winner10920

Yeah they got quite warm, which confused me as well I thought I was spending too much time in linear mode but my oscope confirmed it wasn't
I can't draw a diaragram on my phone but I had the gate directly to the 555 timer output(~4500hz 0-12v supply), the source to my load and the drain to +12v
its the way basically every push pull mosfet diagram I've seen, which isn't how I've seen it as a switch using the drain side but I guess you can't do that in pushpull type method

MarkT

For an n-channel MOSFET you must have the load between positive supply and the drain, connect the source to ground.  You have a source-follower which is ALWAYS in linear mode - it won't be able to pull the output much higher than +9V either.

You are confusing a MOSFET class AB push-pull linear amplifier with a MOSFET switch circuit.  The linear amplifier configuration generates a lot of heat and heatsinks are mandatory because you are in the linear region - this is how you might design an audio amplifier, not a switching application.

For switching first get a decent switching MOSFET (Rds(on) should be low, something more like 0.01 ohm, so a heatsink isn't needed - but calculate this according to your load).  Always use common-source configuration, not source-follower.  If you have to switch on the high-side of the load you need a p-channel device.
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winner10920

Ah ok thatbmakes sense,
That would cause the mosfet to fail u think? It was warm when I touched perhaps it had already burnt out?

winner10920

Could I connect my load to both drains, then have the sources go to v+ and v and it will work without being in linear mode and burning up?

Grumpy_Mike


winner10920

How would I connect it then to get either v+ or v - full on
I guess like hald an hbridge?

Grumpy_Mike

Quote
I guess like hald an hbridge?

If that means half a h-bridge then yes.

winner10920

Yeah typo, srry
so for that how would I configure my fets?

Grumpy_Mike

Configure? You don't configure components.
I assume you mean wire them up?
What sort of FETs, what sort of voltage do you need to switch, what sort of drive do you have?

Generally you need a p-channel and n-channel, along with a few resistors to ensure that the two are not on at the same time.
Do a google image search for some schematics.

winner10920

How's this link work for a shematic, seem correct to you? http://www.cadvision.com/blanchas/hexfet/npch-sw-sch.htm
im gonna use an irf9520 I think i  and an irf 540
My plan to drive it is from a 555 timer, im gonna set up a dual supply opamp to take the 555 output from 0/12v to -12/+12,

dc42

If what you're trying to do is drive a speaker or similar load with a 5KHz square wave, you might be better off using bipolar transistors instead of mosfets. Use a cmos 555 (which drives the output right to the supply rails, unlike the bipolar version), and replace your N-channel source follower with an NPN small signal transistor driving a PNP power transistor (see http://en.wikipedia.org/wiki/Sziklai_pair but also add a resistor between the base and emitter of the PNP transistor). Then add an opposite pair (PNP small signal transistor driving NPN power transistor) to pull the output down to ground. This isn't quite as efficient as using a mosfet half h-bridge (because you will have maybe 1.5v drop across whichever power transistor is turned on), but it avoids the problems of shoot-through that you tend to get with home-made h-bridges.

If the power required in the load is very large, then the mosfet h-bridge is the most efficient solution; but in this case I recommend you buy a mosfet h-bridge driver chip. You won't need any p-channel mosfets because these driver chips are always designed so that both the upper and lower switches in each half h-bridge are n-channel.
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Grumpy_Mike

I don't like that because it is possible to turn both FETs on at the same time.
You should look for a circuit that can't do that even by accident.

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